1. Field of the Invention
The invention relates to surface processing of a crystal in manufacturing a crystal device such as, for example, a tuning fork crystal resonator, a longitudinal mode crystal resonator or crystal sensor, a crystal filter or the like, and relates especially to surface processing of a crystal suitable in the manufacture of crystal pieces in which desired shapes are formed using photolithography technology in the etching process of a crystal wafer.
2. Description of Related Art
Conventionally, crystals (synthetic crystals) are used in various devices such as resonators, generators, filters, sensors and the like, in fields of various electrical appliances including communication equipment such as cellular telephones, pagers, and the like; various electronic control devices and data equipment such as computers, word processors, and the like; and general appliances such as electrical clocks, video cameras, and the like. Among these, tuning fork type crystal resonators are widely used as a clock source of these electrical appliances.
Generally, the manufacture of the tuning fork type crystal resonator comprises a first process of wafer processing in which the rough crystal stone is first cut into a block shape, a wafer of specified thickness is then cut out, and lastly, mirror-like finishing is performed on the surface to achieve a wafer of desired thickness. A process of etching processing then forms the wafer into a tuning fork crystal resonator piece by etching using photolithography technology on which electrodes and wiring patterns are film-formed. Finally, a process is conducted in which the obtained crystal resonator piece is sealed into a vacuum case after being mounted in, for example, a plug comprising a hermetic terminal, and the frequency is adjusted.
In such process of wafer processing, as shown in the flow chart of FIG. 9, after cutting out the crystal wafer, both surfaces of the crystal wafer are lapping processed in order to make the diameter of the grain fine, to remove the cutting processing layer which is generated at that time and to obtain a pre-determined layer thickness. By doing this, the aforementioned surface of the wafer becomes a dull glass-like lap processed surface which has a comparatively rough surface. Next, the layer deteriorated by lapping (or processing-deteriorated layer) is removed and the crystal washed and simultaneously adjusted to the desired thickness through etching of the wafer with ammonium fluoride (NH.sub.4 F) solvent. By doing this, even though extremely fine roughness still exists, a so-called satin-like surface (hereafter, "satin finish surface") can be obtained on the aforementioned wafer surface.
When lapping occurs with a grit of approximately GC#2000, an etching amount of approximately 10 .mu.m is necessary on each wafer surface in order to remove the process-deteriorated layer. However, when the etching amount with NH.sub.4 F is this great, many triangular pyramid-shaped protrusions 3, as seen in FIGS. 10(A) and 10(B), which are called hillocks, are generated on the aforementioned wafer surface corresponding to the direction of the crystal faces of the crystal. These grow larger along with the etching time and at the same time, their numbers are increased. Therefore, the wafer surface cannot be maintained at the same quality as the satin finish surface, and a sufficient quality of the rough surface cannot be obtained in order to form the corrosion film in the later processing and to perform the etching processing of a tuning fork.
Therefore, conventionally, polishing processes are performed by an abrasive material in which very fine grain such as silica and cerium oxide or an etching solution is mixed, and the wafer surface is processed to a high quality mirror surface state by removing hillocks. Moreover, after washing the wafer, it is lightly etched by a mixture solvent of hydrofluoric acid and fluorine ammonium (hereafter, "buffer hydrofluoric acid") (the etching amount of the each surface is approximately 0.5-1 .mu.m), and processing irregularities from the polishing process are removed, and finally, the aforementioned wafer is finished to the desired high quality mirror state having a flatness and a thickness. After that, this wafer is rinsed and finished, and checked for dirt, scratches, and scars. By doing this, at the later etching processing to form the tuning fork, a sufficient amount of adherence is obtained between the wafer surface and the corrosion film which is formed on the wafer surface.
However, in the prior crystal manufacturing method of the tuning fork type crystal resonator piece using surface processing as described above, in the wafer processing process, the time required for the polishing process of the wafer is extremely long and the thickness of the wafer is already thin by etching with the NH.sub.4 F solution. Therefore, the following problems exist. Any damage such as cracking, breaking, or the like easily occurs on the wafer when polishing. The processing work is delicate and troublesome and requires a great amount of work. Simultaneously, the yield decreases and the manufacturing cost increases.